Particle sizing by means of optical techniques using Doppler methods which measure the amount of light scattered by the particle as it passes through a light field is widely practised. For biological particles of diameter 1-10 .mu.m and sub-micron particles the scattered light levels are low. Usually therefore particles are made to flow in a focused light field through a flow cell.
Particular sizing techniques based on Doppler methods require the interferometric combination of crossed laser beams to create a structured pattern. This requires the coherent laser light sources and precision lasers, or more recently, the use of diffraction gratings. The extent of the structured light field necessarily occupies a large part of the inspection volume and consequently requires quality optical components. These requirements are not consistent with the manufacture of a low cost particle sizing equipment.
A problem of known techniques that rely purely on scattered light is that the region of focus is frequently subject to significant variations in radiant intensity when it occupies practical sample volumes. A given particle passing through a region of high focal intensity can therefore scatter the same signal as a larger particle in a region of defocus. It has been proposed to overcome this problem by providing a composite light beam incorporating a concentric trigger of one wavelength through the centre of a surrounding analysis beam having a second wavelength. Only particles whose presence is indicated by the trigger beam are analysed. This occurs when the particle traverses the uniform region of the analysis beam. Measurements of the intensity of scattered light are thus made under repeatable conditions of particle illumination. This technique still requires high precision optics however and does not overcome the difficulty that the intensity of light scattered by particles is not a single valued function of the particle size, so that light intensity does not necessarily provide a reliable indication of particle size.
For example in medical bacteria tests in urine, tests for bacteria (typically 1-4 .mu.m) may not clearly be distinguished from residual red cells (5-8 .mu.) or even white cells (10-15 .mu.m). Additionally still greater expense and complexity is involved.
Thus particle size measurement based solely on the detection of the intensity of the light obscured by a particle as it passes through a uniform focused light field is subject to the major limitation that the signal is a combined function of:
(a) the position of the particle relative to the focal point of the illumination beam PA1 (b) the size, refractive index and absorbtion of the particle.
The first of these limitation means that a large particle passing outside of the focus can generate the same amplitude of obscuration as a small particle passing through the focus. This ambiguity is generally overcome in practice by constraining the flow to the region of focus with the result that the system is vulnerable to blockage and unsuitable for a line measurement. Even under these conditions particles of the same size but having either different refractive indices or absorbtion can give different signals.
The present invention is concerned with reducing the above mentioned disadvantages.